The herpes simplex virus (HSV) genome comprises two covalently linked segments, designated long (L) and short (S). Each segment contains a unique sequence flanked by a pair (terminal and internal) of inverted repeat sequences. The long repeat (RL or RL) and the short repeat (RS or RS) are distinct.
The HSV ICP34.5 (also γ34.5) gene, which has been extensively studied1,6,7,8, has been sequenced in HSV-1 strains F9 and syn17+3 and in HSV-2 strain HG524. One copy of the ICP34.5 gene is located within each of the RL repeat regions. Mutants inactivating both copies of the ICP34.5 gene (i.e. null mutants), e.g. HSV-1 strain 17 mutant 17162 (HSV 1716) or the mutants R3616 or R4009 in strain F5, are known to lack neurovirulence, i.e. be avirulent, and have utility as both gene delivery vectors or in the treatment of tumours by oncolysis. HSV-1 strain 17 mutant 1716 has a 759 bp deletion in each copy of the ICP34.5 gene located within the BamHI s restriction fragment of each RL repeat.
ICP34.5 null mutants of HSV-1 strain 17 have consistently shown much better clinical oncolytic efficacy than mutants in other HSV strains, such as strain F, to the extent that some strain 17 mutants are now in advanced stage clinical trials for the treatment of tumour. Strain 17 ICP34.5 null mutants are additionally advantageous over those of other strains in that they achieve clinical efficacy when administered directly to tumours at dosages that are one or more logs lower than those required to achieve a comparable effect using mutants of other strains.
HSV 1716 is one example of such a mutant and is described in WO 92/139432, specifically incorporated herein by reference. HSV 1716 has been deposited on 28 Jan. 1992 at the European Collection of Animal Cell Cultures, Vaccine Research and Production Laboratories, Public Health Laboratory Services, Porton Down, Salisbury, Wiltshire, SP4 0JG, United Kingdom under accession number V92012803 in accordance with the provisions of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure (herein referred to as the ‘Budapest Treaty’).
HSV1716/CMV-NTR/GFP (referred to herein as HSV 1790) is another exemplary ICP34.5 null mutant of HSV-1 strain 17. This virus is an engineered herpes simplex virus ICP34.5 null mutant which expresses the nitroreductase (NTR) gene and is described in WO 2005/04984514, specifically incorporated herein by reference. It is modified in each ICP34.5 locus by insertion of the E. coli nitroreductase (NTR) gene which disrupts the ICP34.5 protein coding sequence such that the virus lacks a functional ICP34.5 protein. The virus is ICP34.5 deficient, non-neurovirulent and exhibits good oncolytic properties.
In HSV 1790 the NTR gene is operably linked to a transcription control element permitting expression of the NTR gene. As such the virus may be used in gene therapy techniques wherein the virus acts as a vector for the expression of NTR in an HSV infected cell. NTR is capable of converting a range of prodrug molecules, such as CB1954, into cytotoxic active pharmaceutical agents. Thus, HSV1790 can be used in targeted combination therapy in which the oncolytic ability of HSV1790 is combined with localised prodrug activation in tumour cells. HSV 1790 has been deposited (under the name HSV1716/CMV-NTR/GFP) in the name of Crusade Laboratories Limited having an address at Department of Neurology Southern General Hospital 1345 Govan Road Govan Glasgow G51 5TF Scotland on 05 Nov. 2003 at the European Collection of Cell Cultures (ECACC), Health Protection Agency, Porton Down, Salisbury, Wiltshire, SP4 0JG, United Kingdom under accession number 03110501 in accordance with the provisions of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure (herein referred to as the ‘Budapest Treaty’).
To date, the treatment of tumours using ICP34.5 deficient HSV has relied upon direct introduction of the HSV to the tumour, usually by intratumoral injection. This has been considered necessary in order to ensure that the HSV reaches its intended target, i.e. the tumour that is to be treated. Moreover, this approach reduces the risks associated with introduction of a viral vector into healthy tissue in as far as the lytic capacity of the virus is focused on the tumour.
It is well known that tumours may occur in virtually any tissue and at virtually any position in the body. As such it can often be procedurally difficult, as well as the cause of considerable discomfort and possible risk to the patient, to deliver the HSV directly to the tumour. Accordingly, it would be of significant clinical benefit if the oncolytic effect of these HSV could be obtained without having to administer the HSV directly to the tumour. However, the ability of a clinically efficacious oncolytic HSV that is administered to a patient's healthy tissue to successfully and selectively target and lyse tumour tissue located elsewhere in the body, and which does not exhibit serious disadvantageous side-effects on the patient's healthy tissue, remains uncertain.